Electrode catalyst layer for fuel cell

ABSTRACT

An electrode catalyst layer for use in a fuel cell, the layer having a composite particle material in which catalyst particles are supported on conductive particles, a proton conductive polymer and a metal oxide, wherein said metal oxide is non-particulate.

TECHNICAL FIELD

[0001] The present invention relates to an electrode catalyst layer foruse in a solid polymer type fuel cell.

BACKGROUND ART

[0002] A fuel cell is a device which, by electrochemically oxidizinghydrogen, methanol and the like in the cell, converts the chemicalenergy of the fuel directly into electric energy and takes out theenergy; fuel cells are attracting attention as a clean electric energysupply source. In particular, solid polymer type fuel cells are expectedas an alternative source of power for vehicles, a household cogenerationsystem, and an electric power generator for cellular phones since theyoperate at low temperatures as compared to other type cells.

[0003] Such a solid polymer type fuel cell comprises amembrane/electrode assembly (hereinafter referred to as MEA) in which apair of catalyst layers are bonded to both surfaces of a proton exchangepolymer membrane. More specifically, an anode catalyst layer is providedon one surface of the proton exchange polymer membrane, and a cathodecatalyst layer is provided on the other surface of the same membrane.Additionally, an assembly having a structure in which further a pair ofgas diffusion layers placed respectively on both outside surfaces of thecatalyst layers is also referred to as an MEA.

[0004] The anode and cathode catalyst layers have hitherto been sheetsmade of a mixture comprising a carbon black powder made to supportcatalyst particles and a proton conductive polymer; among others, in apreparation method, the catalyst layers are bonded to the protonexchange polymer membrane by heat pressing. Incidentally, an electrodeassembly having a structure in which such catalyst layers are laminatedwith gas diffusion layers is generally referred to as a gas diffusionelectrode.

[0005] A fuel (for example, hydrogen) is supplied to a gas diffusionelectrode as the anode, an oxidant (for example, oxygen and air) issupplied to a gas diffusion electrode as the cathode, and a fuel cellcomes into operation when both electrodes are connected to an externalcircuit. Specifically, when hydrogen is used as fuel, hydrogens areoxidized on the anode catalyst to produce protons. The protons thusproduced pass through the proton conductive polymer portion in the anodecatalyst layer, thereafter migrate through the proton exchange polymermembrane, then pass through the proton conductive polymer portion in thecathode catalyst layer, and thus get onto the cathode catalyst. On theother hand, the electrons generated concurrently with the protons by theoxidation of hydrogens pass through the external circuit and reach thecathode gas diffusion electrode, where the electrons react with theabove described protons and the oxygen in an oxidant to generate water,where electric energy can be taken out.

[0006] The electric power generation performance of a fuel cell largelydepends on the water content regulation in the proton exchange polymermembrane, and in the gas diffusion electrodes of the anode and cathode.Specifically, when the proton exchange polymer membrane is dehumidified,the proton conductance thereof is remarkably decreased and the internalresistance of the cell is increased, resulting in a degradation of theelectric power generation performance.

[0007] Additionally, when the proton conductive polymer portionsconstituting the gas diffusion electrodes of the anode and cathode aredehumidified, the internal resistances of the gas diffusion electrodesare increased, and concurrently the activation overvoltage is increased,resulting in a degradation of the electric power generation performance.In particular, in the anode section, when the protons migrate from theanode section to the cathode section through the proton exchange polymermembrane, the protons are accompanied by water molecules, so that thewater content of the anode section becomes deficient. Consequently, theproton conductive polymer portion in the anode section tends to bedehumidified, and accordingly the proton migration is suppressed to forma water concentration gradient within the proton exchange polymermembrane and thus the decrease of the proton conductance takes place.

[0008] Although, on the other hand, from the viewpoint of simplificationof the fuel cell system, it is preferable to operate the fuel cell underthe condition of low humidification as possible. As described abovethere has been the problem that no satisfactory electric powergeneration performance can be attained unless the proton exchangepolymer membrane and the gas diffusion electrodes in the anode andcathode sections are sufficiently humidified.

[0009] For the purpose of overcoming the above described problem,JP-A-06-111827 proposes a method in which particulate and/or fibroussilica is contained as a water absorbing material in the anode catalystlayer and/or the cathode catalyst layer (hereinafter referred to as themixing method), JP-A-06-111834 proposes a method in which particulateand/or fibrous silica is contained in the proton exchange polymermembrane and JP-A-07-326361 proposes a catalyst layer formed by use of awater absorbing material. By use of these techniques, the water holdingcapacities of the proton exchange polymer membrane and the gas diffusionelectrodes in the anode and cathode sections can be increased, andaccordingly the water content control can be made easier to some extent.

[0010] However, such particulate or fibrous water absorbing materialserves to improve the water holding capacity, but at the same timecauses the increase in electric resistance and the decrease in gaspermeability; thus there is a limit in the effect brought about by sucha material. There is an additional problem that when the amount of sucha material is increased, the catalyst layers and the proton exchangepolymer membrane become brittle, and the catalyst layers cannot bond tothe proton exchange polymer membrane. Accordingly, from a practicalstandpoint, the water content cannot be said to become sufficiently easyto control, such that even when a fuel cell is operated under acondition of low humidification, the effect brought about by such amaterial has been found to be small (see Comparative Examples 2 to 5).

[0011] Incidentally, there has been reported a composite membrane(hereinafter referred to as a sol-gel membrane) in which silica iscontained in a perfluorocarbon based ion exchange membrane by takingadvantage of the sol-gel reaction. Specifically, a perfluorocarbon basedion exchange membrane is soaked and swollen in an aqueous solution of analcohol, such as methanol, and thereafter a mixed solvent comprising atetraethoxysilane (which is a metal alkoxide) and an alcohol is added,and the tetraethoxysilane is subjected to hydrolysis/polycondensationreactions with the aid of the catalytic action of the acidic group.Thus, silica is produced uniformly in the ion exchange membrane (K. A.Mauritz, R. F. Storey and C. K. Jones, in Multiphase Polymer Materials:Blends and Ionomers, L. A. Utracki and R. A. Weiss, Editors, ACSSymposium Series No. 395, P. 401, American Chemical Society, Washington,DC(1989)).

[0012] However, it has been reported that the water holding capacityunder a low humidification condition is only slightly improved, evenwith an increased amount of silica incorporated. Further, the effect ofimproving the water holding capacity is small and additionally theproton conductance is decreased (N. Miyake, J. S. Wainright, and R. F.Savinell, Journal of the Electrochemical Society, 148(8),A898-904(2001)). Accordingly, even when a fuel cell is operated under alow humidification condition, the effect brought about by the sol-gelmembrane has been found to be small (see Comparative Example 6).

DISCLOSURE OF THE INVENTION

[0013] An object of the present invention is to make a satisfactoryelectric power generation performance obtainable even when a fuel cellis operated under a condition of low humidification or humidity.

[0014] The present inventors, as a result of a diligent study for thepurpose of overcoming the above described problems, found that anelectrode catalyst layer, comprising a composite particle material inwhich catalyst particles are supported by conductive particles, a protonconductive polymer, and a metal oxide which is non-particulate, displaysa high water holding capacity and can maintain the proton conductivityat a low humidity. Additionally, the present inventors found thatbecause the above described metal oxide is non-particulate, there are noincrease in electric resistance and no decrease in gas permeability, andaccordingly the electric power generation performance of the fuel cellunder a low humidification condition is improved.

[0015] As a method for obtaining such an electrode catalyst layer, thepresent inventors found a method in which the non-particulate metaloxide is formed, for example, in the following way: after apolymer-containing aggregate containing a composite particle materialand a proton conductive polymer has been formed, a metal oxide precursoris subjected to hydrolysis and polycondensation reactions in the protonconductive polymer. Surprisingly, it has been found that by using thismethod, it has become possible to form the metal oxide in an amountexceeding 100 mass % in relation to the proton conductive polymer, andthe electric power generation performance under a low humidificationcondition is dramatically improved. In other words, the presentinvention is described as follows:

[0016] (1) An electrode catalyst layer for use in a fuel cell, the layercomprising a composite particle material in which catalyst particles aresupported by conductive particles, a proton conductive polymer, and ametal oxide, wherein the above described metal oxide is non-particulate.

[0017] (2) The electrode catalyst layer described in (1), wherein partof the surface of the above described catalyst particles is coated withthe proton conductive polymer.

[0018] (3) The electrode catalyst layer described in (1) or (2), whereinthe above described proton conductive polymer contains the abovedescribed metal oxide.

[0019] (4) The electrode catalyst layer described in any one of (1) to(3), wherein the above described metal oxide is silica.

[0020] (5) The electrode catalyst layer described in any one of (1) to(4), wherein the content of the above described metal oxide is 0.001mg/cm² or more and 10 mg/cm² or less in terms of the loading amount inrelation to the projected area of the electrode.

[0021] (6) The electrode catalyst layer described in any one of (1) to(5), wherein the above described metal oxide is obtained by subjecting ametal oxide precursor to hydrolysis and polycondensation reactions.

[0022] (7) The electrode catalyst layer described in any one of (1) to(6), which is obtained by a method comprising at least the followingsteps of:

[0023] (a) forming a polymer-containing aggregate containing the abovedescribed composite particle material and the above described protonconductive polymer; and

[0024] (b) thereafter converting the above described metal oxideprecursor into the above described metal oxide by impregnating into theaggregate the metal oxide precursor corresponding to the above describedmetal oxide, and successively by subjecting the precursor to thehydrolysis and polycondensation reactions.

[0025] (8) A gas diffusion electrode, comprising the electrode catalystlayer described in any one of (1) to (7).

[0026] (9) A membrane/electrode assembly, comprising the electrodecatalyst layer described in any one of (1) to (7).

[0027] (10) A solid polymer type fuel cell, comprising the electrodecatalyst layer described in any one of (1) to (7).

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a microgram of the surface of an electrode catalystlayer of a blank electrode obtained using a scanning electron microscope(hereinafter referred to as SEM);

[0029]FIG. 2 shows the analysis result 1 (Si2P) for the electrodecatalyst layer produced in Example 1 obtained by means of X-rayphotoelectron spectroscopy (hereinafter referred to as XPS);

[0030]FIG. 3 shows the XPS analysis result 2 (O1s) for the electrodecatalyst layer produced in Example 1;

[0031]FIG. 4 shows the measurements results obtained for the electrodecatalyst layer produced in Example 1 observed along the direction ofthickness thereof by means of an energy dispersive X-ray spectrometer(hereinafter referred to as EDX);

[0032]FIG. 5 is a SEM microgram of the surface of the electrode catalystlayer produced in Example 1;

[0033]FIG. 6 is a microgram of the surface of the electrode catalystlayer produced in Example 1 obtained using a transmission electronmicroscope (hereinafter referred to as TEM);

[0034]FIG. 7 shows the curves for adsorption of the water contentobtained for a gas diffusion electrode and the blank electrode producedin Example 1;

[0035]FIG. 8 is a SEM microgram of the surface of the electrode catalystlayer produced in Example 2; and

[0036]FIG. 9 shows the EDX measurement results obtained for theelectrode catalyst layer produced in Example 2 by observing along thedirection of thickness thereof.

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] Detailed description will be made below on the electrode catalystlayer of the present invention for use in the fuel cell.

[0038] Electrode Catalyst Layer

[0039] The electrode catalyst layer of the present invention for use inthe fuel cell at least comprises an aggregate of a composite particlematerial in which catalyst particles are supported by conductiveparticles, a proton conductive polymer and a non-particulate metaloxide. Although no particular constraint is imposed, it is preferablethat part of the surface of the above described catalyst particles iscoated with the proton conductive polymer, and furthermore it ispreferable that the above described proton conductive polymer containsthe above described metal oxide.

[0040] The conductive particle material can be any type as far as it hasa conductivity; the material used can be carbon black materials such asfurnace black, channel black and acetylene black; activated carbon;graphite; and various metals. The particle size of each of theseconductive particle materials is preferably 10 angstroms or more and 10μm or less, more preferably 50 angstroms or more and 1 μm or less, andmost preferably 100 angstroms or more and 5,000 angstroms or less.

[0041] The catalyst particle material is the catalyst which oxidizes onthe anode the fuel (for example, hydrogen) and makes protons to beeasily produced, and makes on the cathode the protons, electrons andoxidant (for example, oxygen and air) react with each other to generatewater. Although no particular constraint is imposed on the catalysttype, platinum is preferably used. For the purpose of enhancing thetolerance of platinum against impurities, such as CO, preferably usedare the catalysts in which platinum is added or alloyed with rutheniumand the like.

[0042] Although no particular constraint is imposed on the catalystparticle size, the catalyst particle size is preferably 10 angstroms ormore and 1,000 angstroms or less, more preferably 10 angstroms or moreand 500 angstroms or less, and most preferably 15 angstroms or more and100 angstroms or less. The loading amount of the catalyst particlematerial in relation to the projected area of the electrode is, in thestate such that the electrode catalyst layer has been formed, preferably0.001 mg/cm² or more and 10 mg/cm² or less, more preferably 0.01 mg/cm²or more and 5 mg/cm² or less, and most preferably 0.1 mg/cm² or more and1 mg/cm² or less. Additionally, it is typical that such compositeparticles are bonded together to constitute the fundamental skeleton ofan electrode catalyst layer. As the bonding agents, there can be usedfluorocarbon resins such as polytetrafluoroethylene (PTFE) andtetrafluoroethylene-perfluoroalkylvinyl ether copolymer, but protonconductive polymer and the like described below can be also used.

[0043] The proton conductive polymer is a polymer which has protonconductive functional groups. Examples of the proton conductivefunctional groups include a sulfonic acid group, carboxylic acid group,phosphonic acid group, and phosphoric acid group. Examples of thepolymer skeleton include hydrocarbon based polymers such as polyolefinand polystyrene, and perfluorocarbon polymers. Among these polymers,preferable are the perfluorocarbon polymers, which are excellent inresistance to oxidation and heat resistance, represented by thefollowing formula:

—[CF₂CX¹X²]_(a)—[CF₂—CF(—O—(CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (2)

[0044] where X¹, X² and X³ are each independently a halogen or aperfluoroalkyl group having 1 or more and 3 or less carbon atoms, astands for an integer of 0 or larger and 20 or smaller, b stands for aninteger of 0 or larger and 8 or smaller, c is 0 or 1, d, e and f areeach independently integers of 0 or larger and 6 or smaller (with aconstraint such that d+e+f is different from 0), g is an integer of 1 orlarger and 20 or smaller, R¹ and R² are each independently a halogen, ora perfluoroalkyl group having 1 or more and 10 or less carbon atoms, ora fluorochloroalkyl group having 1 or more to 10 or less carbon atoms,and X⁴ is COOH, SO₃H, PO₃H₂ or PO₃H.

[0045] No particular constraint is imposed on the equivalent weight EW(the number of grams in dryness of the proton conductive polymer inrelation to one equivalent of the proton exchange group) of the protonconductive polymer, but the equivalent weight is preferably 500 or moreand 2,000 or less, more preferably 600 or more and 1,500 or less, andmost preferably 700 or more and 1,200 or less. No constraint is imposedon the content of the proton conductive polymer made to be present inthe electrode catalyst layer, but in terms of the loading amount inrelation to the projected area of the electrode and in the state suchthat the electrode catalyst layer has been formed, the content ispreferably 0.001 mg/cm² or more and 10 mg/cm² or less, more preferably0.01 mg/cm² or more and 5 mg/cm² or less, and most preferably 0.1 mg/cm²or more and 2 mg/cm² or less. Additionally, the content is, in terms ofthe mass ratio in relation to the loading amount of the catalystparticle material, preferably 0.001 or more and 50 or less, morepreferably 0.1 or more and 10 or less, and most preferably 0.5 or moreand 5 or less.

[0046] No particular constraint is imposed on the metal oxide,preferable is an inorganic material comprising as a constituent one typeof compound selected from the group consisting Al₂0₃, B₂0₃, MgO, SiO₂,SnO₂, TiO₂, V₂O₅, WO₃, Y₂O₃, ZrO₂, Zr₂O₃ and ZrSiO₄. More preferable areAl₂O₃, SiO₂, TiO₂ and ZrO₂, among which silica (SiO₂) is particularlypreferable. Incidentally, such metal oxide materials generally have —OHgroups on the surface thereof, in such a way that sometimes the case ofSiO₂is given a representation that

SiO_(2(1-0.25X))(OH)_(X)(0≦X≦4)

[0047] The metal oxide does not take a particulate form or a fibrousform, but is present in the electrode catalyst layer in anon-particulate form. In other words, even when observed with an opticalmicroscope and an electron microscope, neither particulate nor fibrousmetal oxide is observed. In particular, even when the electrode catalystlayer is observed at a magnification of a few hundreds of thousandstimes using a scanning electron microscope (SEM), neither particulatenor fibrous metal oxide is observed. Additionally, even when theelectrode catalyst layer is observed at a magnification of a fewhundreds of thousands times to a few millions times using a transmissionelectron microscope (TEM), neither particulate nor fibrous metal oxidecan be clearly observed. As described above, within the scope of theexisting microscopic techniques, the particles of the above describedmetal oxide cannot be verified.

[0048] No constraint is imposed on the content of the metal oxide, butin terms of the loading amount in relation to the projected area of theelectrode and in the state such that the electrode catalyst layer hasbeen formed, the content of the metal oxide is preferably 0.001 mg/cm²or more and 10 mg/cm² or less, more preferably 0.01 mg/cm² or more and 5mg/cm² or less, furthermore preferably 0.1 mg/cm² or more and 3 mg/cm²or less, and most preferably 0.5 mg/cm² or more and 3 mg/cm² or less. Noconstraint is imposed on the mass ratio of the metal oxide to the protonconductive polymer, but the mass ratio is preferably 0.001 or more and50 or less, more preferably 0.01 or more and 20 or less, furthermorepreferably 0.1 or more and 5 or less, and most preferably 1 or more and5 or less.

[0049] The electrode catalyst layer of the present invention may containadditives such as a conducting agent, a bonding agent and awater-repellent agent. No constraint is imposed on the conducting agent,as far as the conducting agent is an electron conductive material, andexamples of the conducting agent include carbon black such as furnaceblack, channel black and acetylene black; activated carbon; graphite;and various metals. Examples of the bonding agent and/or thewater-repellent agent include fluorocarbon resins such aspolytetrafluoroethylene (PTFE) andtetrafluoroethylene-perfluoroalkylvinyl ether copolymer.

[0050] Now, a description will be made below on the method for producingthe electrode catalyst layer of the present invention.

[0051] The electrode catalyst layer of the present invention can beproduced by use of the so-called sol-gel reaction and by takingadvantage of the pH dependence of the rates of the hydrolysis andpolycondensation reactions of the metal oxide precursor. For example,the hydrolysis reaction of the metal oxide precursor under an acidiccondition proceeds extremely faster than that under a neutral conditionowing to the catalytic effect of the acidic condition, so that the metaloxide precursor is selectively subjected to the hydrolysis andpolycondensation reactions in the neighborhood of the proton exchangegroups in the proton conductive polymer, in particular, in theneighborhood of the so-called ion cluster in which a plurality of protonexchange groups are associated with each other. Accordingly, the metaloxide, produced by the hydrolysis and polycondensation reactions of themetal oxide precursor, is present in the proton conductive polymer inextremely fine forms, but does not take particulate forms or fibrousforms.

PREPARATION EXAMPLE 1

[0052] The present preparation example comprises the step of forming apolymer-containing aggregate containing at least a composite particlematerial and a proton conductive polymer (hereinafter referred to aspolymer-containing aggregate forming step) and the step of converting ametal oxide precursor corresponding to the above described metal oxideinto said metal oxide by impregnating, into the polymer-containingaggregate, the metal oxide precursor and successively by subjecting themetal oxide precursor to the hydrolysis and polycondensation reactions(hereinafter referred to as metal oxide forming step).

[0053] (Polymer-Containing Aggregate Forming Step)

[0054] As a method for forming a polymer-containing aggregate containingat least a composite particle material and a proton conductive polymer,generally known methods can be used. Among these methods is thefollowing method.

[0055] At the beginning, a catalyst dispersion is prepared whichcontains at least the composite particle material and the protonconductive polymer. No constraint is imposed on the medium for thecatalyst dispersion; examples of the medium include single componentmediums such as water, lower alcohols such as ethanol, ethyleneglycol,propyleneglycol, glycerin, dimethylsulfoxide, and composite mediumscontaining two or more of these mediums. In this connection, such adispersion medium may contain a bonding agent, a water-repellent agent,a conducting agent and the like. By applying the dispersion thusobtained onto an ion exchange polymer membrane, a gas diffusion layer orother substrates (PTFE membrane or the like) and by drying itthereafter, the polymer-containing aggregate can be formed on thesubstrates.

[0056] No constraint is imposed on the type of the ion exchange polymermembrane; however, perfluorocarbon polymer is preferable similarly tothe case of the above described proton conductive polymer. No constraintis imposed on the membrane thickness, but it is preferably 1 μm or moreand 500 μm or less. Examples of the gas diffusion layer includeconductive porous woven and nonwoven cloths such as carbon paper andcarbon cloth. The polymer-containing aggregate formed on the substratesuch as a film made of PTFE may be transcribed or bonded to the ionexchange polymer membrane by heat pressing or the like.

[0057] Alternatively, the polymer-containing aggregate can be formed asfollows: a dispersion containing at least the proton conductive polymeris prepared and applied onto or impregnated into an aggregate composedof the composite particles supporting the catalyst particles thereon,and then dried to form the polymer-containing aggregate. The aggregateas referred to here is compatible with a gas diffusion electrode, thetypical example of which is gas diffusion electrode ELAT® manufacturedby E-TEK, Inc., USA.

[0058] (Metal Oxide Forming Step)

[0059] The polymer aggregates, produced as described above on varioussubstrates, are impregnated with the metal oxide precursor and theimpregnated precursor is successively subjected to hydrolysis andpolycondensation reactions.

[0060] No constraint is imposed on the types of the metal oxideprecursor to be used in the present invention, but preferable arealkoxides containing Al, B, P, Si, Ti, Zr or Y, among which alkoxidescontaining Al, Si, Ti or Zr are particularly preferable. Specificexamples of the alkoxide of Al include Al(OCH₃)₃, Al(OC₂H₅)₃, Al(OC₃H₇)₃and Al(OC₄H₉)₃; specific examples of the alkoxide containing B includeB(OCH₃)₃; specific examples of the alkoxide containing P includePO(CH₃)₃ and P(OCH₃)₃; specific examples of the alkoxide containing Siinclude Si(OCH₃)₄, Si(OC₂H₅)₄, Si(OC₃H₇)₄ and Si(OC₄H₉)₄; specificexamples of the alkoxide containing Ti include Ti(OCH₃)₄, Ti(OC₂H₅)₄,Ti(OC₃H₇)₄ and Ti(OC₄H₉)₄; specific examples of the alkoxide containingZr include Zr(OCH₃)₄, Zr(OC₂H₅)₄, Zr(OC₃H₇)₄ and Zr(OC₄H₉)₄; andspecific examples of the alkoxide containing Y include Y(OC₄H₉)₃. Thesemay be used each alone or in combinations of two or more thereof.Additionally, the following alkoxides containing two types of metals maybe used: La[Al(i-OC₃H₇)₄]₃, Mg[Al(i-OC₃H₇)₄]₂, Mg[Al(sec-OC₄H₉)₄]₂,Ni[Al(i-OC₃H₇)₄]₂, (C₃H₇O)₂Zr[Al(OC₃H₇)₄]₂ and Ba[Zr₂(OC₂H₅)₉]₂.

[0061] The impregnation amount of the metal oxide precursor is notlimited, but is, in relation to 1 equivalent of the proton exchangegroup in the proton conductive polymer, preferably 0.01 equivalent ormore and 1,000,000 equivalents or less, more preferably 0.05 equivalentor more and 500,000 equivalents or less, most preferably 0.1 equivalentor more and 100,000 equivalents or less, and furthermore preferably 0.2equivalent or more and 20,000 equivalents or less.

[0062] The amount of water to be used for soaking the proton conductivepolymer for the purpose of hydrolysis is not limited, but is, inrelation to 1 equivalent of the metal oxide precursor, preferably 0.1equivalent or more and 100 equivalents and less, more preferably 0.2equivalent or more and 50 equivalents or less, most preferably 0.5equivalent or more and 30 equivalents or less, and furthermorepreferably 1 equivalent or more and 10 equivalents or less.

[0063] The metal oxide precursor or water may be added after having beendiluted with or dissolved in another solvent. No constraint is imposedon the procedure for initiating the hydrolysis and polycondensationreactions; examples of the initiation procedure include a procedure inwhich at the beginning water is impregnated into the polymer-containingaggregate and thereafter the metal oxide precursor is added; a procedurein which the metal oxide precursor is impregnated into thepolymer-containing aggregate and thereafter water is added; and aprocedure in which a liquid containing both water and the metal oxideprecursor is impregnated into the polymer-containing aggregate. Whenpracticing these procedures, the metal oxide precursor and water may bediluted with or dissolved in another solvent, and then added.

[0064] No constraint is imposed on the reaction temperature forperforming the hydrolysis and polycondensation reactions, but thereaction temperature is preferably 1° C. or above and 100° C. or below,more preferably 10° C. or above and 80° C. or below, and most preferably20° C. or above and 50° C. or below. No constraint is imposed on thereaction time, but the reaction time is preferably 1 second or longerand 24 hours or shorter, more preferably 10 seconds or longer and 8hours or shorter, and most preferably 20 seconds or longer and 1 hour orshorter.

[0065] By imposing the above described conditions on the metal oxideprecursor, usually at the beginning, the hydrolysis reaction of themetal oxide precursor takes place, and then the polycondensationreaction proceeds.

[0066] After a predetermined period of time, the polymer-containingaggregate is taken out from the liquid, the liquid adhering to thesurface thereof is removed and/or washed out according to need, andthereafter the aggregate is allowed to stand in the air at 1 to 80° C.Subsequently, according to need, the aggregate is subjected to a heattreatment at 80 to 150° C. under a dry condition and/or to a hot watertreatment at 80 to 150° C., and thus the electrode catalyst layer of thepresent invention can be obtained.

PREPARATION EXAMPLE 2

[0067] The present preparation example includes the step in which themetal oxide precursor is added to the solution containing at least thecomposite particle material and the proton conductive polymer, thesolution is mixed, and thus the catalyst dispersion binder solution isprepared (catalyst dispersion binder solution preparation step), thestep in which the metal oxide precursor is subjected to the hydrolysisand polycondensation reactions (metal oxide forming step), and the stepin which the solvent is evaporated from the catalyst dispersion bindersolution after the hydrolysis step and thus the binder solution issolidified to form the electrode catalyst layer (electrode catalystlayer forming step).

[0068] (Catalyst Dispersion Binder Solution Preparation Step)

[0069] At the beginning, a catalyst dispersion containing at least thecomposite particle material and the proton conductive polymer isprepared. No constraint is imposed on the type of the medium for thedispersion solution; examples of the medium include single componentmediums such as water, lower alcohols such as ethanol, ethyleneglycol,propyleneglycol, glycerin, dimethylsulfoxide, and composite mediumscontaining two or more of these mediums. In this connection, such adispersion may contain a bonding agent, a water-repellent agent and aconducting agent.

[0070] A metal oxide precursor, similar to that in Preparation Example1, is added to and mixed in the dispersion to produce the catalystdispersion binder solution. No constraint is imposed on the amount ofthe metal oxide precursor, but the preferable added amount of the metaloxide precursor is similar to that in Preparation Example 1. Theaddition of the metal oxide precursor is performed sometimes with themetal oxide precursor alone, and in some other cases with the metaloxide precursor dissolved in or diluted with another solvent. In thecase where the dispersion thus produced contains water, the hydrolysisand polycondensation reactions are made to start at the same instant oftime when the metal oxide precursor is added.

[0071] (Metal Oxide Forming Step)

[0072] When the catalyst dispersion binder solution is nonaqueous or thewater content thereof is small, the dispersion is added with water andstirred, and thus the hydrolysis and polycondensation of the metal oxideprecursor is started. At this time, water diluted with another solventcan also be added. No constraint is imposed on the amount of water; thepreferable added amount of water, preferable reaction temperature andpreferable reaction time are the same as those described for PreparationExample 1. The metal oxide precursor is not necessarily exhausted in thehydrolysis and polycondensation reactions; some of the precursor mayremain unreacted.

[0073] (Electrode Catalyst Layer Forming Step)

[0074] The catalyst dispersion binder solution obtained in the metaloxide forming step is applied onto a variety of substrates, and then thesolvent is evaporated and the applied binder solution is solidified. Asthe substrates, similarly to those described in Preparation Example 1,an ion exchange polymer membrane, a gas diffusion layer and othersubstrates such as PTFE film can be used.

[0075] Thereafter, according to need, the coated substrates are allowedto stand in the air at 1 to 80° C. Subsequently, according to need, thesubstrates are subjected to a heat treatment at 80 to 150° C. under adry condition and/or a hot water treatment at 80 to 150° C., and thusthe electrode catalyst layers according to the present invention can beobtained. When the electrode catalyst layer is formed on a substratesuch as a film made of PTFE, the electrode catalyst layer may betranscribed and bonded to an ion exchange polymer membrane by heatpressing and the like, to form an electrode catalyst layer on the ionexchange polymer membrane.

PREPARATION EXAMPLE 3

[0076] The present preparation example includes the step in which themetal oxide precursor is added to the solution containing at least thecomposite particle material and the proton conductive polymer, thesolution is mixed, and thus the catalyst dispersion binder solution isprepared (catalyst dispersion binder solution preparation step), thestep in which the solvent is evaporated from the catalyst dispersionbinder solution and thus the binder solution is solidified to form thepolymer-containing aggregate (polymer-containing aggregate forming step)and the step in which the metal oxide precursor is subjected to thehydrolysis and polycondensation reactions (metal oxide forming step).

[0077] (Catalyst Dispersion Binder Solution Preparation Step)

[0078] At the beginning, a dispersion containing the composite particlematerial and the proton conductive polymer is produced. No constraint isimposed on the type of the medium for the dispersion; examples of themedium include single component mediums such as lower alcohols such asethanol, ethyleneglycol, propyleneglycol, glycerin, dimethylsulfoxide,and composite mediums containing two or more of these solvents. Thewater content of the dispersion is recommended to be as small aspossible, and is preferably 1 ppm or more and 5 mass % or less, morepreferably 1 ppm or more and 1 mass % or less, and most preferably 1 ppmor more and 1,000 ppm or less. The dispersion may contain a bondingagent, a water-repellent agent and a conducting agent.

[0079] A metal oxide precursor is added to and mixed in the dispersionto produce the catalyst dispersion binder solution. The metal oxideprecursor to be used is similar to that described in PreparationExample 1. No constraint is imposed on the amount of the metal oxideprecursor, but the preferable added amount of the metal oxide precursoris similar to that in Preparation Example 1. When added, the metal oxideprecursor may be used alone, or may be used after having been dilutedwith or dissolved in another solvent.

[0080] (Polymer-Containing Aggregate Forming Step)

[0081] The catalyst dispersion binder solution produced as describedabove is applied onto a variety of substrates, and then the solvent isevaporated and the applied binder solution is solidified. As thesubstrates, similarly to those described in Preparation Examples 1 and2, an ion exchange polymer membrane, a gas diffusion layer, and othersubstrates such as PTFE membrane can be used. When thepolymer-containing aggregate is formed on a substrate such as a filmmade of PTFE, the aggregate may be transcribed and bonded to an ionexchange polymer membrane by heat pressing and the like.

[0082] (Metal Oxide Forming Step)

[0083] The polymer-containing aggregates produced on the varioussubstrates as described are soaked in water, and the hydrolysis andpolycondensation reactions of the metal oxide precursor are made tostart. At this time, water diluted with another solvent may also beadded. No constraint is imposed on the amount of water; the preferableadded amount of water, preferable reaction temperature and preferablereaction time are the same as those described for Preparation Example 1.

[0084] After a predetermined period of time, the polymer-containingaggregate is taken out from the liquid, the liquid adhering to thesurface thereof is removed and/or washed out according to need, andthereafter the aggregate is allowed to stand in the air at 1 to 80° C.Subsequently, according to need, the aggregate is subjected to a heattreatment at 80 to 150° C. under a dry condition and/or a hot watertreatment at 80 to 150° C., and thus the electrode catalyst layer of thepresent invention can be obtained.

[0085] (Gas Diffusion Electrode)

[0086] When the electrode catalyst layer of the present invention isused in a solid polymer type fuel cell, the electrode catalyst layer isgenerally used as a gas diffusion electrode having a form in which a gasdiffusion layer made of a conductive porous woven or nonwoven cloth,such as carbon paper and carbon cloth, is bonded onto or laminated onthe electrode catalyst layer.

[0087] (MEA)

[0088] When the electrode catalyst layer of the present invention isused in a solid polymer type fuel cell, the electrode catalyst layer isused as an MEA in which at least two types of catalyst layers, namely acathode catalyst layer and an anode catalyst layer, are respectivelybonded to the two surfaces of a proton exchange polymer membrane. Theelectrode catalyst layer of the present invention is applied to eitherone of the anode and cathode, or to both of the anode and cathode. Noconstraint is imposed on the type of the proton exchange polymermembrane, but preferable is an ion exchange membrane made of aperfluorocarbon polymer similar to the above described proton conductivepolymer. Incidentally, an assembly having a structure in which further apair of gas diffusion layers placed respectively on both outsidesurfaces of the catalyst layers is also referred to as an MEA.

[0089] When the electrode catalyst layer of the present invention isformed on a proton exchange polymer membrane, the membrane can be used,as it is, as the MEA of the present invention. When the electrodecatalyst layer of the present invention is formed on an ion exchangepolymer membrane other than the proton type, it is necessary to convertthe ion exchange polymer membrane into a proton type membrane by soakingthe membrane into an acid, such as hydrochloric acid, so that the MEA ofthe present invention may be obtained. When the electrode catalyst layerof the present invention is formed in a gas diffusion electrode or on agas diffusion layer, the MEA of the present invention can be produced bybonding the electrode catalyst layer to an ion exchange polymer membraneby heat pressing and the like. Similarly to the case described above,when the ion exchange polymer membrane is not of the proton type, it isnecessary to convert the ion exchange polymer membrane into a protontype membrane by further soaking the membrane into an acid, such ashydrochloric acid. When the electrode catalyst layer of the presentinvention is formed on any other substrate (a film made of PTFE and thelike), the MEA of the present invention can be produced by transcribingand bonding the electrode catalyst layer to the ion exchange polymermembrane with the aid of heat pressing and the like. When the ionexchange polymer membrane is not of the proton type, it is necessary toconvert the ion exchange polymer membrane into a proton type membrane byfurther soaking the membrane into an acid, such as hydrochloric acid.

[0090] Fuel Cell

[0091] The solid polymer type fuel cell is composed of the MEA of thepresent invention, a gas diffusion layer, a bipolar plate, a backingplate and the like. Among these, the bipolar plate is a plate made ofgraphite, a composite material composed of graphite and a resin, or ametallic material which has grooves on the surface thereof for flowing agas of a fuel, an oxidant or the like, and is provided with thefunctions for transmitting electrons into an external circuit andadditionally working as flow paths for supplying the fuel and theoxidant to the neighborhoods of the gas diffusion layer and the catalystlayer. Lamination of a plurality of such bipolar plates and a pluralityof MEAs intervening between the bipolar plates results in formation of afuel cell. The operation of the fuel cell is performed eventually bysupplying hydrogen to one electrode and oxygen or air to the otherelectrode.

[0092] The operation temperature of the fuel cell is usually 50° C. orabove and 80° C. or below, in which range the water content can beeasily controlled (although sometimes the fuel cell is operated at 100°C. or above and 150° C. or below because the higher the temperature is,the higher the catalyst activity is). The inside cell pressures ofhydrogen and oxygen are preferably as high as possible because thereactivity is increased and hence the output power of the fuel cell isimproved. But from the view point of the durability of the MEAmaterials, it is preferable to regulate these pressures so as to fallwithin appropriate pressure ranges, respectively.

[0093] The electrode catalyst layer, gas diffusion layer and MEA of thepresent invention can find applications in chlor-alkali, waterelectrolysis, hydrohalogenic acid electrolysis, sodium chlorideelectrolysis, oxygen concentrators, temperature sensors, gas sensors andthe like.

[0094] Now, specific description will be made below on the presentinvention on the basis of the examples, but the present invention is notlimited by the examples. The evaluation method measurement methods andanalysis methods adopted in the present invention are as describedbelow.

[0095] (Fuel Cell Evaluation)

[0096] An MEA was produced as follows: the gas diffusion electrode inthe anode section and the gas diffusion electrode in the cathode sectionwere made to face each other, a sheet of a perfluorosulfonic acidmembrane (manufactured by Asahi Kasei Corp.) of 950 in EW and 50 μm inthickness was sandwiched therebetween, and the laminate thus obtainedwas subjected to hot pressing at 150° C. under an applied pressure of 50kg/cm², to yield the MEA.

[0097] The MEA was set in a single cell evaluation apparatus as the fuelcell, and a single cell characteristics test was conducted by usinghydrogen gas as fuel and air as oxidant under atmospheric pressure, at acell temperature of 80° C. and a current density of 0.5 A/cm². The waterbubbling method was used for gas humidification, and the air washumidified at room temperature and then supplied to the cell. When thehumidification temperature for the hydrogen gas was set at 60° C., thecell voltage and the cell internal resistance obtained by the currentinterrupt method were measured.

[0098] (Observation through SEM)

[0099] A system S-4700 (manufactured by Hitachi, Ltd.) was used forconducting microscopic observation of the individual samples. Theindividual samples were cut to appropriate sizes, placed on a samplestage, and subjected to Os coating to form samples for microscopicobservation for the purpose of the surface analysis of the electrodecatalyst layer.

[0100] (EDX Measurement)

[0101] Individual samples were cut to appropriate sizes and embedded inan epoxy resin, and then subjected to cutting with the aid of anultramicrotome; the cut samples each were placed on a sample stage witha mirror surface made in the direction of thickness of the electrodecatalyst layer, and were subjected to carbon deposition to preparesamples for microscopic observation along the direction of thickness.While each of the samples for microscopic observation was subjected toobservation through SEM, the X-ray intensities ascribable to the elementSi (originating from SiO₂), element S (originating fromperfluorosulfonic acid polymer), and element Pt (originating from thecatalyst particles) were measured by means of an X-ray analyzerEMAX-7000 (manufactured by Horiba, Ltd.), and thus the distributions ofthe elements along the direction of thickness were investigated.

[0102] (Observation through TEM)

[0103] Individual samples were subjected to microscopic observation bymeans of a system H-7100 (manufactured by Hitachi, Ltd.). Individualsamples were cut to appropriate sizes and embedded in an epoxy resin,and then subjected to cutting with the aid of an ultramicrotome intoultrathin specimens, which were observed at an acceleration voltage of125 kV.

[0104] (XPS Measurement)

[0105] An X-ray photoelectron spectroscopic analysis was performed byuse of a system PHI 5400 (manufactured by Physical Electronics Inc.).The individual samples were cut to 5 mm dices and the electrode catalystlayer surfaces were subjected to measurement as they were. Themeasurements conditions included the use of the Mg Kα line as anexcitation X-ray, the output power of 15 kV×26.6 mA, and the analysisarea of 3.5 mm×1 mm. The acquisition region for the conducted surveyscan covered the range from 0 to 1,100 eV, and narrow scan was appliedto the Si2p and O1s regions. The pass energy for the survey scan was setat 178.9 eV, and that for the narrow scan was set at 35.75 eV.

[0106] (Adsorbed Water Content Measurement)

[0107] Measurement of the adsorbed water content was conducted by meansof an apparatus BELSORP 18 (manufactured by Bel Japan, Inc.). On thebasis of the so-called volume method, the adsorption amounts of water toa sample at 30° C. for different relative humidities were derived fromthe gas pressure variations within the system by use of the equation ofstate for gas.

EXAMPLE 1

[0108] A 5 mass % perfluorosulfonic acid polymer solution (manufacturedby Asahi Kasei Corp., EW: 910, solvent composition (by weight):ethanol/water=50/50) was applied onto a gas diffusion electrode ELAT®(Pt loading of 0.4 mg/cm², which is the same hereinafter, too)manufactured by E-TEK, Inc., USA., thereafter dried and immobilized at120° C. in the air atmosphere, and thus an electrode catalyst layer witha polymer loading of 0.8 mg/cm² was produced. The electrode catalystlayer at this stage was used as a blank electrode. FIG. 1 shows a SEMmicrogram of the surface of this electrode catalyst layer of the blankelectrode. According to this microgram, the electrode catalyst layer isfound to be wholly formed of the aggregate of the composite particles inwhich Pt fine particles as the catalyst particles are supported onconductive carbon particles, and at least part of the surface of thecomposite particles is found to be covered with the perfluorosulfonicacid polymer layer.

[0109] The blank electrode was subjected to the following treatments.Specifically, the above described blank electrode was soaked, at roomtemperature for 1 hour, in a methanol/water mixed solution (1 ml/unitarea (cm²) of the catalyst sheet) which had been beforehand prepared ina volume ratio of 1:1, thereafter a tetraethoxysilane/methanol mixedsolution ((3 ml/unit area (cm²) of the catalyst sheet) which had beensimilarly beforehand prepared in a volume ratio of 3:1 was poured intothe above described methanol/water mixed solution. The solution thusobtained was further stirred and mixed for 1 minute. Thus, silica wasformed in the electrode catalyst layer. Immediately thereafter, thetreated electrode was taken out, fully washed with methanol, dried atroom temperature for 3 hours, thereafter dried in the air at 120° C. for1 hour, and thus a gas diffusion electrode constituted with theelectrode catalyst layer of the present invention was produced. The massdifference between before and after the above described treatmentrevealed that silica was loaded in 2.54 in terms of the mass ratio inrelation to the perfluoro-sulfonic acid polymer. The surface of theelectrode catalyst layer was subjected to XPS measurement. FIG. 2 showsthe results obtained from narrow scan of the Si2p region, and FIG. 3shows the results obtained from narrow scan of the O1s region; thepositions of these peaks confirming the presence of silica.Additionally, as a result of the EDX measurement, as FIG. 4 shows, Siwas also detected from the depth of the electrode catalyst layer.

[0110]FIG. 5 shows a SEM microgram of the surface of the electrodecatalyst layer. According to FIG. 5, the surface of the electrodecatalyst layer of the present invention is the same as that of the blankelectrode, and neither particulate nor fibrous silica was observed.Furthermore, FIG. 6 shows a TEM microgram. The conductive carbonparticles and Pt catalyst particles were observed, but neitherparticulate nor fibrous silica was observed.

[0111]FIG. 7 shows the results obtained from the adsorbed water contentmeasurement on the gas diffusion electrode of the present invention andthe blank electrode; as can be confirmed from FIG. 7, the gas diffusionelectrode of the present invention exhibited a higher water content thanthe blank electrode, and additionally maintained a high water contenteven at a low humidity.

[0112] A fuel cell with the anode and cathode both adopting the gasdiffusion electrode of the present invention was evaluated, thusobtaining a cell voltage of 0.54 V and a cell internal resistance of0.10 Ωcm². The results thus obtained are shown in Table 1.

EXAMPLE 2

[0113] A fuel cell with the anode adopting the gas diffusion electrodeof the present invention and the cathode adopting the blank electrodewas evaluated, thus obtaining a cell voltage of 0.51 V and a cellinternal resistance of 0.11 Ωcm². The results thus obtained are shown inTable 1.

EXAMPLE 3

[0114] A fuel cell with the anode adopting the blank electrode and thecathode adopting the gas diffusion electrode produced in Example 1 wasevaluated, thus obtaining a cell voltage of 0.51 V and a cell internalresistance of 0.11 Ωcm². The results thus obtained are shown in Table 1.

EXAMPLE 4

[0115] To 0.519 g of a Pt-supporting carbon (TEC10E40E, 36.4 wt % Pt,manufactured by Tanaka Kikinzoku Co., Ltd.) as a composite particlematerial, 3.78 g of a perfluorosulfonic acid polymer solution was added,and the mixture thus obtained was fully mixed together by means of ahomogenizer. The dispersion thus obtained was applied onto a sheet of acarbon paper (TGP-H-060, manufactured by Toray Industries, Inc.) byscreen printing, dried at room temperature for 1 hour and then in theair at 140° C. for 1 hour, and thus a polymer-containing aggregate ofthe order of 10 μm in thickness was obtained. In the polymer-containingaggregate, the Pt loading was 0.3 mg/cm², the polymer loading was 0.3mg/cm², and the gas diffusion electrode thus obtained was used as areference electrode.

[0116] The reference electrode was subjected to the following treatment.Specifically, the above described electrode was soaked, at roomtemperature for 1 hour, in a methanol/water mixed solution (1 ml/unitarea (cm²) of the catalyst sheet), which had been beforehand prepared ina volume ratio of 1:3, thereafter a tetraethoxysilane/methanol mixedsolution (3 ml/unit area (cm²) of the catalyst sheet), which had beensimilarly beforehand prepared in a volume ratio of 3:1, was poured intothe above described methanol/water mixed solution, the solution thusobtained was further stirred and mixed for 5 seconds, and thus silicawas formed in the polymer-containing aggregate. Immediately thereafter,the treated electrode was taken out, fully washed with methanol, driedat room temperature for 3 hours, thereafter dried in the air at 120° C.for 1 hour. Thus, a gas diffusion electrode constituted with theelectrode catalyst layer of the present invention was produced. The massdifference between before and after the above described treatmentrevealed that the silica loading was 0.3 mg/cm², and the loading amountwas 1.00 in terms of the mass ratio in relation to the perfluorosulfonicacid polymer.

[0117] A fuel cell with the anode and cathode both adopting the gasdiffusion electrode was evaluated, thus revealing that the electricpower generation characteristics were better than the characteristics ofa cell with the anode and cathode both adopting the reference electrode,and the cell voltage was improved by 0.066 V at the current of 0.5A/cm².

COMPARATIVE EXAMPLE 1

[0118] A fuel cell with the anode and cathode both adopting the blankelectrode was evaluated, thus obtaining a cell voltage of 0.43 V and acell internal resistance of 0.18 Ωcm².

COMPARATIVE EXAMPLE 2

[0119] A dispersion was prepared in which a fine silica particlematerial (Aerosil® 380, manufactured by Japan Aerosil Co., Ltd., averageprimary particle size 0.007 μm) was dispersed in a 5 mass % solution ofperfluorosulfonic acid polymer in such a way that the mass ratio betweenthe polymer and silica was 3:1. The dispersion thus prepared was appliedonto a gas diffusion electrode ELAT® (manufactured by E-TEK, Inc.,USA.), thereafter dried at room temperature for 3 hours and then driedat 120° C. in the air for 1 hour. Thus, a gas diffusion electrodeconstituted with a catalyst layer based on a conventional method wasobtained. The mass difference between before and after the applicationof the dispersion revealed that the fine silica particle material(Aerosil® 380, manufactured by Japan Aerosil Co., Ltd.) was loaded in2.55 in terms of the mass ratio in relation to the perfluorosulfonicacid polymer. FIG. 7 shows an electron microgram of the surface of thiselectrode catalyst layer. As shown in FIG. 7, there is observed thecondition that the surface of the catalyst layer is wholly covered witha layer of the silica fine particles. As the EDX measurement results inFIG. 8 show, silica was revealed to be unevenly distributed near thesurface.

[0120] With the anode and cathode both adopting this gas diffusionelectrode, production of an MEA was unsuccessfully attempted such thatthe catalyst layers were not transcribed to the two surfaces of themembrane, respectively.

COMPARATIVE EXAMPLE 3

[0121] A gas diffusion electrode constituted with the catalyst layersbased on a conventional method was produced in a manner similar to thatin Comparative Example 2, except that the silica fine particle materialwas loaded in 0.08 in terms of the mass ratio in relation to theperfluorosulfonic acid polymer. According to the electron microscopicobservation, similarly to Comparative Example 2, there was observed thecondition that the surface of the catalyst layer was covered with asilica fine particle layer.

[0122] A fuel cell with the anode and cathode both adopting the gasdiffusion electrode was evaluated, thus obtaining a cell voltage of 0.47V and a cell internal resistance of 0.18 Ωcm².

COMPARATIVE EXAMPLE 4

[0123] To 0.519 g of a Pt-supporting carbon (TEC10E40E, 36.4 wt % Pt,manufactured by Tanaka Kikinzoku Co., Ltd.) as a composite particlematerial, 7.56 g of a perfluorosulfonic acid polymer solution and 0.019g of the same silica fine particle material as was used in ComparativeExample 1 were added, and the mixture thus obtained was fully mixedtogether by means of a homogenizer. The dispersion solution thusobtained was applied onto a sheet of PTFE by screen printing, dried atroom temperature for 1 hour, thereafter dried in the air at 120° C. for1 hour, and thus a catalyst layer of the order of 10 μm in thickness wasobtained. In the catalyst layer, the Pt loading was 0.4 mg/cm², thepolymer loading was 0.8 mg/cm², and the silica loading was 0.04 mg/cm²and 0.05 in terms of the mass ratio in relation to the perfluorosulfonicacid polymer.

[0124] A fuel cell with the anode and cathode both adopting the catalystlayer was evaluated, thus revealing that the performance was poor, thevoltage fluctuation was large, and additionally the current density wasnot able to be elevated up to 0.5 A/cm². At a current density of 0.2A/cm², the cell voltage was found to be 0.36 V and the cell internalresistance was found to be 0.39 Ωcm². The results obtained are shown inTable 1.

COMPARATIVE EXAMPLE 5

[0125] A catalyst layer was produced in a manner similar to that inComparative Example 4, except that the dispersion was prepared by adding0.960 g of the silica fine particle material; the Pt loading was 0.4mg/cm², the polymer loading was 0.8 mg/cm², and the silica loading was2.03 mg/cm² and 2.54 in terms of the mass ratio in relation to theperfluorosulfonic acid polymer.

[0126] A fuel cell with the anode and cathode both adopting the catalystlayer was evaluated, thus revealing that the performance was poor, thevoltage variation was large, and additionally even a current of 0.1A/cm² was not able to be obtained. The results obtained are shown inTable 1.

COMPARATIVE EXAMPLE 6

[0127] A sheet of a perfluorosulfonic acid membrane (manufactured byAsahi Kasei Corp.) of 950 in EW and 50 μm in thickness was dried in avacuum oven at 100° C. for 24 hours. The membrane sample was soaked for1 hour in a methanol/water mixed solution (1 cm³/unit area of themembrane (cm²)) prepared in a volume ratio of 2:1, thereafter atetraethoxysilane/methanol mixed solution (3 cm³/unit area of themembrane (cm )) prepared in a volume ratio of 3:2 was poured into theabove described methanol/water mixed solution, the solution thusobtained was further stirred and mixed for 1 minute. Thus, silica wasformed in the membrane. Then, the membrane was dried in a vacuum oven at100° C. for 24 hours. The mass variation between before and after theabove described treatment revealed that silica was loaded in 0.05 interms of the mass ratio in relation to the perfluorosulfonic acidmembrane. The silica composite membrane thus obtained was sandwichedbetween two blank electrodes and subjected to hot pressing at 150° C.under an applied pressure of 50 kg/cm², to yield an MEA. A fuel cellevaluation conducted by use of the MEA revealed that the performance waspoor such that the cell voltage was 0.33 V and the cell internalresistance was 0.22 Ωcm². TABLE 1 Cell Internal MEA voltage resistanceAnode Cathode Ion exchange membrane V Ω cm² Example 1 Present PresentPerfluorosulfonic acid 0.54 0.10 invention invention membrane Example 2Present Blank Perfluorosulfonic acid 0.51 0.11 invention membraneExample 3 blank Present Perfluorosulfonic acid 0.51 0.11 inventionmembrane Comparative blank Blank Perfluorosulfonic acid 0.43 0.18Example 1 membrane Comparative Conventional ConventionalPerfluorosulfonic acid MEA production Example 2 method method membraneimpossible Comparative Conventional Conventional Perfluorosulfonic acid0.47 0.18 Example 3 method method membrane Comparative ConventionalConventional Perfluorosulfonic acid Limiting current Example 4 methodmethod membrane density: 0.2 A/cm² Comparative Conventional ConventionalPerfluorosulfonic acid Limiting current Example 5 method method membranedensity: 0.1 A/cm² Comparative Silica composite 0.33 0.22 Example 6membrane

[0128] Industrial Applicability

[0129] The use of the electrode catalyst layer of the present inventionmakes it possible to stably operate the fuel cell and improve theelectric power generation performance even under a condition of lowhumidification.

1. An electrode catalyst layer for use in a fuel cell, the layercomprising a composite particle material in which catalyst particles aresupported on conductive particles, a proton conductive polymer and ametal oxide, wherein said metal oxide is non-particulate.
 2. Theelectrode catalyst layer according to claim 1, wherein part of thesurface of said catalyst particles is coated with the proton conductivepolymer.
 3. The electrode catalyst layer according to claim 1 or 2,wherein said proton conductive polymer contains said metal oxide.
 4. Theelectrode catalyst layer according to any one of claims 1 to 3, whereinsaid metal oxide is silica.
 5. The electrode catalyst layer according toany one of claims 1 to 4, wherein the content of said metal oxide is0.001 mg/cm² or more and 10 mg/cm² or less in terms of loading amount inrelation to projected area of the electrode.
 6. The electrode catalystlayer according to any one of claims 1 to 5, wherein said metal oxide isobtained by subjecting a metal oxide precursor to hydrolysis andpolycondensation reactions.
 7. The electrode catalyst layer according toany one of claims 1 to 6, which is obtained by a method comprising atleast the following steps of: (a) forming a polymer-containing aggregatecontaining said composite particle material and said proton conductivepolymer; and (b) thereafter converting a metal oxide precursorcorresponding to said metal oxide into said metal oxide by impregnatinginto the aggregate said metal oxide precursor and successively bysubjecting the precursor to the hydrolysis and polycondensationreactions.
 8. A gas diffusion electrode, comprising the electrodecatalyst layer according to any one of claims 1 to
 7. 9. Amembrane/electrode assembly, comprising the electrode catalyst layeraccording to any one of claims 1 to
 7. 10. A solid polymer type fuelcell, comprising the electrode catalyst layer according to any one ofclaims 1 to 7.